While many advances in transplantation biology have been made possible by the availability of numerous inbred strains of mice, there remains a need for comparable large animal models for organ transplantation. Over the past three years we have developed three strains of miniature swine, each homozygous for a different allele of the major histocompatibility locus (MHC). To accomplish this in a minimum number of generations, a selective breeding scheme was used. Prior to breeding, the original parents were reciprocally immunized with skin grafts and lymphocyte injections. This produced antisera used in a microcytotoxicity assay with appropriate absorptions to type the offspring of this mating. Selective breeding based on serologic typing of the 62 descendants of this original pairing has led to the development of three separate herds, each homozygous at the MHC. Using hyperimmune sera raised by immunizations between members of these three homozygous strains, 138 different cytotoxicity combinations were tested on lymphocytes from 2nd and 4th generation pigs. No reactions inconsistent with serologic specificities predicted genetically were obtained, confirming the segregation of serologically defined alleles. Mixed lymphocyte cultures likewise confirmed the serologic genotyping: between 68 pairs of animals tested, the mean two-way stimulation index was 1.00 plus or minus 0.07 for serologically identical animals vs. 9.59 plus or minus 2.94 for animals with one or two haplotype differences.